With the introduction of the damascene process, the formation of wires by filling trenches has become routine. Additionally, particularly for the case of copper-filled trenches, the method of choice for depositing the metal has been electroplating. However, as trenches have become narrower and narrower, it has become necessary to employ a range of additives that need to be included in the plating solution to give the best filling results. The technology involved is further complicated by the fact that the effects of these multiple additives are often interactive.
We can identify three broad additive types:
Accelerators serve to increase the deposition rate during electroplating. They are usually small organic molecules containing a polar sulfur, oxygen, or nitrogen functional group. In addition to increasing the deposition-rate, they promote denser nucleation which leads to the growth of films having a finer grain structure. Accelerators are usually present in the plating bath at a low concentration level (1–25 ppm).
Suppressors are additives that reduce the plating rate and are usually present in the plating bath at higher concentrations (200–2,000 ppm), so that their concentration at the interface is not strongly dependent on their rate of mass transfer or diffusion to the wafer surface. They are generally polymeric surfactants with high molecular weight such as polyethylene glycol (PEG). The suppressor molecules slow down the deposition rate by adsorbing on the wafer surface where they form a diffusion barrier.
Levelers are additives whose purpose is to reduce surface roughness. They are similar to suppressors in that they reduce deposition rate. However, they are present in very small concentrations (<25 ppm) so their blocking effects at the surface are highly localized. The net effect is that they selectively reduce deposition on the high spots thereby giving the low spots a chance to ‘catch up’.
It has been known for some time that narrow trenches (typically having widths less than about 1 microns) tend to fill more rapidly than wide trenches (typically having widths greater than about 2 microns). This results in problems of the type schematically illustrated in FIG. 1. Seen there, in cross-section, is a portion of a substrate (typically a silicon wafer) 11 in whose upper surface several narrow trenches 12 and one wide trench 13 have been formed. After electro-deposition of metal layer 14 the latter is found to have the profile shown, i.e. it locally thicker over the narrow trenches and locally thinner over the wide trench.
The general approach that the prior art has taken to dealing with this problem has been to try to balance the concentrations of the various additives so as to find a single formulation that works well for both narrow and wide trenches simultaneously. As will be shown, the present invention has abandoned this approach in favor of a two-step plating method.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,346,479 B1, Woo et al. show an electroplating process of first filling holes using an electroplating process that has been optimized for conformal coating followed by a second electroplating step that has been optimized for non-conformal coating. Trench width is not explicitly taught as a criterion for determining which solution to use where.
Chen et al., in U.S. Pat. No. 6,207,222 B1, show multi-step plating to fill a Cu dual damascene opening while U.S. Pat. No. 6,140,241 (Shue et al.), U.S. Pat. No. 6,136,707 (Cohen), and U.S. Pat. No. 5,814,557 (Venkatranman) all show related plating processes.